1. Field of the Invention
This invention generally relates to automatic extrusion die constructions, especially for forming generally flat sheet or cast film polymer webs. More specifically, the present invention relates to automatic extrusion die constructions in which a plurality of individual, selectively controllable pressure developing thrust elements is supported from one lip of the die at points spaced longitudinally therealong and with each thrust element operative to apply a variable amount of thrust to the corresponding lip area in order to flex that lip area toward and away from the other lip and thereby selectively narrow or widen the die lip opening for a more uniform extruded resin thickness.
2. Description of Related Art
Current technology for automatic extrusion dies involves the use of either threaded bolts turned by servomotors, or by the use of electric heaters in a heat actuated expandable cartridge assembly in order to mechanically manipulate the flexible lip of a sheet die to accomplish gauge control of polymers. The automatic die works in conjunction with a downstream gauge scanning mechanism which gives a feedback signal via a microprocessor to individually actuate bolts or heat expandable cartridges of the autodie. In order to reduce areas of high gauge, the corresponding servomotors turn down the threaded bolts or apply power to the heat expandable cartridges in order to mechanically close the lip of the die in the corresponding areas in question. In the areas of low gauge the servomotors retract the threaded bolt, or cut off electrical power to the heat expandable lip cartridge to relax mechanical pressure on the die lip and thus allow natural relaxation of the die lip in response to internal pressure of the polymer and the die.
The state of the current technology is indicated by the disclosures of the following patents. U.S. Pat. No. 3,901,636, to Stanley C. Zink et al. discloses a plastic film extrusion apparatus including two melt chambers heated to different temperatures. The two heated chambers are each used to heat the resin of the thermoplastic material extruded through that chamber and the specific temperatures to which the chambers are heated are maintained above and below the temperature of oxidation of the resin of the thermoplastic material extruded through the die. The use of two melt chambers at temperatures below and above the oxidation temperature of the resin allows two webs or films to be extruded from the die with the high temperature film sandwiched between the low temperature film and a substrate, trapping the odor of the high temperature film.
U.S. Pat. No. 4,781,562, to Takayosyhi Sano et al. discloses heat actuated thrust units operatively associated with a die lip. Each thrust unit functions to increase the width of the die slot when the heat actuated thrust unit is activated and to decrease the width of the die slot when the heat actuated thrust unit is deactivated.
U.S. Pat. No. 5,229,140, to Guenther Crass et al. discloses a lip die having mechanical thrust developing structures associated with a flexible lip and also an elongated bar extending longitudinally of the flexible lip including selectively operable heaters spaced along the bar, the bar being adjustably reciprocated along the flexible die lip to achieve uniform heating of the die lip.
U.S. Pat. No. 5,401,454, to Kilian K. Mendel discloses an extrusion die and flow control method incorporating a flow control in the form of a flow diverter which extends transversely across the flow path.
U.S. Pat. No. 5,423,668, to Peter F. Cloeren discloses a thermally controlled lip gap mechanism or adjustment incorporating continuous heating and heat sink sources in an attempt to more quickly adjust a die lip gap or slit as deviation in produced web thickness occurs downstream from the die.
Finally, U.S. Pat. No. 5,487,655, to Detlef Frey et al. discloses an apparatus for producing a spun fiber fleece which insures a highly uniform or homogenous distribution of the mechanical properties described in the web and which allows, in particular, the elimination of detrimental local inhomogeneities.
It is also known in blown film applications having circular dies to use heaters in the air ring which sits atop the blown film die. The air from the air ring is normally employed to cool the bubble of blown film as it is extruded and stretched. The heaters selectively apply heat to the air directed at areas of thicker film thickness as controlled by a down stream gauge scanner. The added heat allows the film to continue to stretch and become thinner than it would without the heated air applied in the thicker film areas.